Optical component with spectral separation

ABSTRACT

The invention relates to an optical component comprising at least one input monomode fiber ( 1 - 4 ), at least one output monomode fiber ( 5 ) and a diffractive element ( 7 ) which is disposed between the input fiber or fibers ( 1 - 4 ) and the output fiber or fibers ( 5 ). The inventive component is characterized in that at least one of the input or output fibers ( 1 - 5 ) comprises a fiber ( 1 - 5 ) containing a portion ( 21 - 25 ) which is designed to increase the radius of the mode field it guides. According to the invention, the portion which is designed to increase the mode field radius can comprise a portion with a graded index, a portion having a core or cladding refractive index which varies transversely and/or longitudinally.

BACKGROUND OF THE INVENTION

The invention concerns the field of optical components and moreparticularly optical wavelength multiplexers and demultiplexers.

The person skilled in the art knows that it is possible to considerablyincrease the traffic of fiber optic networks through wavelengthmultiplexing and demultiplexing techniques. Each series of data to betransported is transmitted over a specific optical frequency,multiplying the capacity of the fiber by the number of wavelengths used.

FIG. 1 represents a multiplexer of the prior art. In this type ofmultiplexer, basic optical fibers 1 to 4 each dedicated to a frequencyband have their end in a plane x constituting the input plane of themultiplexer. This multiplexer also comprises a collimation element 6 anda diffractive element 7. The input plane x of the multiplexer isconfused with the focal plane of the collimation element 6 so that theinput beams coming from the ends of the basic fibers 1 to 4 pass throughthe collimation element 6 and are located roughly parallel to oneanother. The diffraction element 7 is positioned so that the beams aresent back toward the collimation element 6, which superimposes them tointroduce them at the end of a single output fiber 5.

As the rate of optical networks continues to increase and as thestability of the optical sources, particularly lasers, is not perfect,it is necessary to reduce the transmission fluctuations resulting fromthis instability by seeking a multiplexing bandwidth that is as wide aspossible.

We therefore want to increase the ratio FWHM/Δλ where FWHM (Full Widthat Half Maximum) designates the width of a basic band and Δλ designatesthe distance between two central wavelengths of two consecutive basicbands. We know that the ratio FWHM/Δλ is proportional to the ratio ω/Δλ,where ω designates the mode field radius of the fibers and Δx designatesthe spacing between the fibers.

BRIEF DESCRIPTION OF THE INVENTION

One known method for increasing the ratio FWHM/Δλ therefore consists inusing a planar wave guide-based concentrator to reduce Δx. Thisintegrated optical component in effect makes it possible to reduce thephysical space between the paths. This type of concentrator isparticularly well suited for use in planar wave guide AWG (Array WaveGuide Grating) multiplexers/demultiplexers. However, it is relativelyexpensive.

Another method consists in increasing ω. Thus, document EP 0 859 249describes a fiber optics multiplexer comprising basic input fibers eachcarrying a frequency band and an array of microlenses, each microlensbeing associated with one fiber end. These microlenses make it possibleto converge the beams coming from the basic fibers to produce parallelbeams with a larger mode field radius than that of the input beams. Thebeams pass through a collimation lens that directs them toward adispersion grating making it possible to generate a single output beamconsisting of different superimposed beams.

It will be understood that this technique requires precise positioningof the ends of the fibers with respect to the focal points of themicrolenses as well as precise alignment of the axes of the basic fiberswith respect to the focal axes of the microlenses.

One goal of this invention is to provide an optical fibermultiplexer/demultiplexer that is more economical, easy to assemble andhas an enhanced FWHM/Δλ ratio.

To this end, the invention proposes an optical component comprising atleast one input monomode fiber, at least one output monomode fiber and adiffractive element positioned between the input fiber or fibers and theoutput fiber or fibers, characterized in that at least one of the inputor output fibers comprises a fiber comprising a portion designed toincrease the mode field radius it guides.

Within the framework of this invention, the portion designed to increasethe mode field radius of the beam may be formed of a portion with agraded index.

This type of optical component advantageously makes it possible toobtain a beam whose mode field radius is increased with respect to themode field radius carried by the associated guiding element.

In this component, the mode field radius expansion function of each beamis advantageously integrated in the input fiber and/or output fiber ofthis beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become clear upon reading thefollowing description, which is purely illustrative and non-limiting innature and which must be read in light of the appended figures.

FIG. 1 is a schematic representation of a multiplexer/demultiplexer witha diffractive element of the prior art,

FIG. 2 is a schematic representation of a multiplexer/demultiplexer thatconforms to a mode of embodiment of the invention,

FIG. 3 represents a fiber example comprising a portion with a gradedindex.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Of course this invention is not limited to the particular number offibers illustrated in the appended figures, particularly to a 4 to 1multiplexer, but extends to any component comprising n fibers.

The component represented in FIG. 2 operates like a multiplexer (thistype of component could, of course, also be used as a demultiplexer).This component comprises coplanar optical fibers 1 to 5 that areparallel to one another and juxtaposed. Fibers 1 to 4 are input fibers,each dedicated to a given frequency band. Fiber 5 is an output fiberensuring the transmission of the multiplexed optical beam obtained bysuperimposing the beams coming from the input fibers 1 to 4. Thecomponent also comprises a focusing element 6 of the lens type placedopposite the ends of fibers 1 to 5 and a diffractive element 7, forexample a diffraction grating, that receives the signals coming from theinput fibers 1 to 4 via the focusing element 6.

In known manner, the diffraction element 7 has the property of sendingback the different wavelengths contained in the same incident beamseparated angularly. Pursuant to the principle of light return, thegrating can recombine the incident beams separated angularly and comingfrom the input fibers 1 to 4 in the direction of the output fiber 5 viathe focusing element 6.

The input fibers 1 to 4 and the output fiber 5 have respectively attheir end a silica portion 11 to 14 and 15 as well as an optical fiberportion with graded refractive index 21 to 24 and 25. The portions ofgraded refractive index fiber have a core whose refractive index variesbased on the radial distance. The refractive index, higher at the centerof the core, decreases as we approach the optical cladding, thus forcingthe light rays to follow a curved trajectory that periodically refocuseson the central axis of the core. In a graded refractive index opticalfiber, the refractive index of the fiber changes according to adetermined continuous refractive index variation law, for exampleparabolic. Thus, the tilted ray moving away from the axis encounters arefractive index environment that decreases progressively which lays itdown and brings it back toward the axis.

In this figure, the beams coming from the graded index portions travelin the void up to the focusing element 6, which focuses them on theelement 7. Each beam corresponds to a given frequency range, and thegrating is adapted to superimpose all the incident beams into a singlebeam directed toward the focusing element 6 and the output fiber 5.

In FIG. 3, we have illustrated more specifically an example of fiber 1comprising a graded refractive index portion. This type of fiber isformed of a classic monomode fiber 31 at the end of which has beenspliced a silica segment 11 of length Ls followed by a graded refractiveindex fiber segment 21 with length Lg constituting the graded refractiveindex portion. The beams coming from the core of the monomode fiber 31successively pass through the pure silica segment 11 and the gradedrefractive index segment 21. In the pure silica segment 11, the beamshave a tendency to diverge while in the graded refractive index silicasegment 21, they have a tendency to reconcentrated. The working distancez_(ω) and the mode field radius ω of the beam leaving the fiber dependon the lengths Ls and Lg of the segments 11 and 21 spliced to themonomode fiber 31.

It is also possible to use similar fibers that do not comprise the puresilica segment 11. In this case, the monomode fiber 31 is directlyspliced to the graded refractive index fiber segment 21.

In one implementation of the multiplexer of FIG. 2, the input fibers 1to 4 and the output fiber 5 consists of monomode fibers 31 to 35 with aconstitution similar to the one in FIG. 3.

The basic fibers 1 to 5 of FIG. 2 can be positioned in a fiber holdercomprising V-shaped grooves for positioning the fibers. The ends offibers 1 to 5 are then polished to be aligned with one another. Thepolishing operation slightly modifies the length of the graded-indexfiber portions 21 to 25. We can show that this modification in lengthhas few consequences for the mode range ω of the beam leaving the fiber.

Nevertheless, to precisely control the behavior of the beam, it ispossible to add an additional silica segment at the end of each fiber 1to 5 without any effect on the trajectory of the beams. The fibers arethen positioned in the fiber holder with this additional segment beforebeing polished together. In this way, the length of the graded-indexportions 21 to 25 is not altered by the polishing operations.

We can also obtain enhanced performance by reducing the diameter of thefibers at their end. To this end, we can advantageously produce achemical attack of their external surfaces to remove a layer of theoptical cladding. We thus decrease the value of the spacing Δx betweenthe mode field radii.

Furthermore, the invention makes it possible to eliminate the alignmentproblems inherent in the prior art.

Previously we described components according to the invention in whichthe portion designed to increase the mode field radius of the beam isformed of a portion with a graded refractive index. However, theinvention is not limited to this particular mode of embodiment. Asindicated previously, this invention also extends to the case where theportion ensuring the mode increase is formed of a portion of fiber whosecore or cladding size varies longitudinally and/or transversally or evenof a portion whose core or cladding refractive index varieslongitudinally and/or transversally.

Additionally, it will be understood that the invention is not limited tothe modes of embodiment previously described in which the diffractiveelement consists of a diffraction grating. For example, the diffractiveelement may consist of an echelle grating, a volume-phase holographicgrating, a prism, or even the combination of several of these elements.

1. An optical component comprising at least one input monomode fiber(1-4), at least one output monomode fiber (5), a diffractive element (7)positioned between the input fiber or fibers (1-4) and the output fiberor fibers (5), and a focusing element (6) placed, between the ends ofthe fibers (1-5) and the diffractive element (7), characterized in thatat least one of the input or output fibers (1-5) comprises agraded-index portion (21-25) designed to increase the radius of the modefield it guides for an enhancement of the FWHM/Δλ ratio and that thefocusing element (6) is separated from the fibers.
 2. The component asclaimed in claim 1, wherein the portion (21-25) designed to increase themode field radius is formed of a segment(s) of fiber added and splicedto the end of the fiber (1-5).
 3. The component as claimed in claim 1,wherein the component forms a wavelength multiplexer/demultiplexer. 4.The component as claimed in claim 3, wherein the diffractive element (7)receives the incident beams from a plurality of input fibers (1-4), thediffractive element (7) combines those incident beams into a single beamand sends this single beam to at least one output fiber (5).
 5. Thecomponent as claimed in claim 3, wherein the diffractive element (7)receives an incident beam containing different wavelengths from at leastone input fiber (5), the diffractive element (7) separates the incidentbeam angularly and sends the beams separately to output fibers (1-4). 6.The component as claimed in claim 1, wherein the fiber or fibers (1-5)also comprise a pure silica portion positioned between the end of thefiber (31) and the fiber portion ensuring the increase in the mode fieldradius (21-25).
 7. The component as claimed in claim 1, wherein eachfiber (1-5) comprising a portion designed to increase the mode fieldradius has at its end a protective silica portion connected to the fiberportion ensuring the increase of the mode field radius (21-25), saidprotective silica portions can be polished without the risk of modifyingthe length of the fiber portion ensuring the increase of the mode fieldradius (21-25).
 8. The component as claimed in claim 1, wherein thediameter of the end of the fiber or fibers (1-5) comprising at least onefiber portion ensuring the increase of the mode field radius (21-25) isreduced by chemical attack of the external surfaces of the fibers. 9.The component as claimed in claim 1, wherein the diffractive element (7)is chosen from the group consisting of echelle gratings, volume-phaseholographic gratings, prisms or a combination of these elements.
 10. Thecomponent as claimed in claim 1, wherein the focusing element is a lensspatially separated from the input and output fibers.